Closed loop interactive controller

ABSTRACT

An injection mold apparatus has multiple injection zones, each zone having at least one heater and at least one temperature sensor generating a temperature indicating signal. A power source provides power to the heaters. A controller controls the temperature of at least some of the zones. For efficiency, the controller has two separate processors, a data-receiving processor for receiving temperature indicating signal from each sensor as well as power signals, and a control processor for receiving data from the data-receiving processor and for controlling the amount of power provided to the heaters. Preferably, the control is in a housing, with the housing mounted directly on the mold. Modified PID calculations are utilized. Power calculations for the amount of power to the heaters utilizes a modulo based algorithm.

COPYRIGHT NOTICE

A portion (Appendices A-1, 2, 3, and 4) of this patent document containsmaterial which is subject to copyright protection. The copyright ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent files or records, but otherwise reserves allcopyright rights whatsoever.

BACKGROUND

This invention relates to a controller integrated with apparatus for thepurpose of controlling the operational parameters of the apparatus, andmore particularly, to control circuitry operated by processors for aclosed loop, self-remedial system. A particular application of thepresent invention relates to molds utilized with a plastic moldapparatus, such as a hot runner injection mold apparatus.

In the injection mold industry, it is known to have an injection moldapparatus having from one to two hundred plus molding stations. Moldshaving ninety-six stations are common. Each station is equipped with thefunctional elements necessary for carrying out the molding process,including a mold cavity, resin feed equipment, and heaters and chillingfluid channels for maintaining resin and the mold in a proper moldingcondition. In a typical injection mold process, the following stepssequentially take place: (1) the mold cavity closes; (2) molten resin isinjected into the closed mold; (3) once the injected resin hardens, themold is opened; and (4) a formed part is ejected from the opened mold.Each mold cavity can undergo changes in spacing as a result of repeatedopening and closing as each finished molded component is ejected. It iscritical that the mold return to its precise spacing after thecomponents are ejected and the mold is readied for the next batch.Moreover, in molding components, it is critical to maintain thetemperature of the resin within a few degrees of its desired melttemperature in order to achieve the proper quality of the moldedcomponents. Given the high production rate of injection moldingapparatus, almost instantaneous recognition that the system is out oftemperature control is needed.

Conditions in the mold that are important in terms of product qualityare the resin temperature, the mold temperature, and mold pressure. Ifthese conditions are out of set ranges, the quality of the final moldedcomponents can be adversely affected. For example the molded componentscan emerge with extraneous plastic along the edges, which is known as“flash”.

Other problems incurred with molding parts are variation andinconsistency in the weight and quality of the components.

Conventional techniques for controlling operational parameters of a moldto maintain the parameters within a range of acceptable referenceparameters involves monitoring the various parameters with sensors. Thedata detected by the sensors is transmitted as analog signals to acontroller. Certain examples of the prior art are U.S. Pat. No.5,551,857, issued Sep. 3, 1996, to Osami Fujioka, which disclosescontrols for a molding apparatus 39 (FIG. 1); and U.S. Pat. No.5,795,511, issued Aug. 18, 1998, to Peter G. Kalantzis, which disclosesa memory function in FIG. 2 for the operational parameters of the hotside 24 of all injection molding machine.

Some of the disadvantages of prior art techniques for monitoring andcontrolling the operational parameters of a mold apparatus areattributable to the use of large, stand alone controllers, external tothe mold. These controllers are expensive, and require large kilowattpower sources and large heavy cables for connection. The cables thatprovide the kilowatt power generates resistance in the cables andproduces unwanted noise that can result in inaccurate signals from thevarious sensors. Moreover, a large number of connectors need to be madeto connect the mold controller to (i) the mold, (ii) the injection moldapparatus, and (iii) a power source. Effecting these connections delaysstart-up of the equipment, and can contribute to high labor costs forproduction of molded parts.

In view of the disadvantages of prior art techniques, there is a needfor control apparatus, such as for an injection mold apparatus, andmethods for operating the apparatus, that permit accurate, automatic andinexpensive control of operational parameters while minimizingproduction of defective parts.

SUMMARY

The present invention is directed to an apparatus and method thatsatisfy this need. In particular, in one aspect of the presentinvention, an apparatus has multiple zones, each zone having at leastone heater and/or chiller system, and at least one temperature sensoroutputting a temperature indicating signal. A power source providespower to any heater, chilling fluid is provided to any chiller system,and a controller controls the temperature of at least some of the zones.The controller comprises a data-receiving processor and a separatecontrol processor. The data-receiving processor receives a temperatureindicating signal from each sensor. The separate control processorreceives data from the data-receiving processor and controls powerprovided to the heaters and/or the temperature of chilling fluidprovided to the chilling system in response to the data received fromthe data-receiving processor. Each processor has its own centralprocessing unit.

Typically the apparatus is a hot runner injection mold having multipleinjection zones. Typically the controller comprises a closed loop feedback circuit. Preferably the data-receiving processors also calculatethe root mean square current of each heater and/or root mean square ofthe output temperature of fluid from the chiller.

To avoid the problem of the expensive and error-inducing cables,preferably the controller is in a housing, where the housing is mountedon the mold. If necessary, there can be an insulating air gap betweenthe housing and the mold to avoid heat from the mold overheating theprocessors.

In order to avoid production of defective parts, preferably theapparatus comprises an alarm responsive to an out of control conditionsuch as incomplete ejection of a molded component from the mold, out ofspecification component quality, irregular spacing between moldcomponents, and incorrect weight of molded components. The apparatus cancomprise an automatic or manual control switch responsive to the alarmfor shutting down any zone responsible for the out of control condition.

The apparatus typically utilizes a power source providing AC current tothe heaters. The control processor compares the actual temperature ofeach zone against its target temperature. The control processor providesan output signal for regulating the power source, and controls the totalnumber of complete current cycles provided by the power source to eachheater. This is accomplished by calculating the number of current cyclesrequired to achieve the target temperature based on the differencebetween actual temperature and target temperature, and then comparingthe number of current cycles needed against the actual number of cyclesbeing provided to each heater. For this purpose, the apparatus includesa detector, such as a transformer, for detecting the amount of currentprovided to each heater.

The data-receiving processor and control processor preferably are onseparate printed circuit boards. Because the data-receiving function ofdata received from the heaters and the current sensors isprocessor-intensive for a large number of zones, the data-receivingprocessor can comprise separate processor modules, each module havingits own CPU. For example, for 96 zones each module can be used for 48zones.

In a preferred control processor, the amount of power needed for eachheater is determined with a PID calculation, where a range of limits isprovided for the amount the power to be applied, independent of the PIDcalculation to prevent over heating the system.

To minimize the calculation load on the first processor, preferably theRMS value for detected current utilizes an algorithm to calculate thesum of squares which requires less processor capacity, as detailedbelow.

To be certain, for power efficiency, that AC power is provided incomplete cycles, the system includes a zero cross-over detector andcontrols for starting and ending power application at zero cross-over.

Optionally, the present invention monitors each molding station with asensor during the resin injection step. If the sensor detects that themold is improperly opening during the injection step, which can causeflash, the system automatically increases the pressure utilized to keepthe mold closed.

The present invention also includes a system and method for starting upthe apparatus, so that all zones reach a startup target temperature atsubstantially the same time. This is effected by determining a heat-uptime for each zone, where the heat-up time is equal to the amount oftime required to heat up the adjacent portion of the apparatus to itsstart-up temperature with the heater on substantially full power. Theheat-up times of the heaters are compared to identify the longestheat-up time. The amount of heat-up power for each heater is determined,the heat-up power for each zone being the amount required to heat up therespective adjacent portions of the apparatus to its start-uptemperature in the heat-up time. Then there is applied to each heatersimultaneously its respective heat-up power, with the result that allportions of the apparatus reach their target temperature atsubstantially the same time.

In a more general sense, a system according to the present inventioncontrols the operation of an apparatus having a plurality of operatingzones, where the system comprises at each zone, at least one operatingelement (such as a heater), and a detector (such as a transformer)generating an operating signal representative of the operating state ofthe operating element (such as power to the heater). An input isprovided for each operating element to vary an operational parameter(such as temperature) of each zone. A sensor senses the operationalparameter of each zone and generates a corresponding analog sensingsignal. The system includes a controller that comprises a firstprocessor for digitizing the signals and determining from the signal theoperating state of the operating element and the operational parameterof each zone. The controller also includes a second processor forperforming a PID calculation for each zone based on the operating stateand the operational parameter to determine the amount of input of eachoperating element to maintain the operational parameter of each zonewithin a selected range. The second processor preferably limits themaximum amount of input determined by the PID calculation to preventdamage to the apparatus and operating elements.

As a result of the combination of these features of the presentinvention, including separate processors for data-receiving and control,a controller mounted directly on the apparatus without cables, anefficient algorithm for calculating RMS values, use of full AC cyclesfor power to the heaters, and a system for detecting out of controlconditions, it is possible to efficiently, effectively, andautomatically manufacture molded parts.

DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood from the following description,appended claims, and accompanying drawings where:

FIG. 1 is a perspective view of a mold apparatus according to thepresent invention, showing an interactive process manager (IPM)integrated with a mold having multiple zones.

FIG. 2 is a schematic view of the inside of the mold of FIG. 1, takenalong line 2—2 in FIG. 1.

FIG. 3 is a sketch illustrating techniques for controlling andmonitoring the condition of the mold of FIG. 1 and the condition ofmolded components for the mold of FIG. 1.

FIG. 4 is a block diagram showing the modules of the IPM and therelationship between modules of the IPM of the apparatus of FIG. 1.

FIG. 5 is block diagram of the temperature and current sensing datamodules of the IPM of FIG. 1.

FIG. 6 is a software flow chart of the computations carried out formaking decisions on power requirements for heaters of the mold of FIG.1.

FIGS. 7A and 7B are top and bottom plan views, respectively, of a powerdriver unit of the IPM of FIG. 1 showing the arrangement of thecomponents that form a two-sided circuit board.

FIGS. 7C and 7D are sectional views of the power driver unit of FIGS. 7Aand 7B, taken on lines 7C—7C and 7D—7D, respectively, in FIG. 7A.

FIG. 7E is a wiring diagram of the power driver unit of FIGS. 7A and 7Bthat senses the current that is input to a processor module of the IPM.

FIG. 8 is a block diagram of a processor module that is part of the IPMof FIG. 1 that controls the timing of the application of the calculatedpower to mold zones.

FIG. 9 is a flow chart of software that controls the triggering ofapplication of power for heating the mold zones of the mold of FIG. 1.

FIG. 10 is a flow chart of the software that controls the powerincrements applied to each heater of the mold of FIG. 1 in bringing thezones to the operational reference temperature.

FIG. 11A is a time vs. temperature plot of the start-up technique ofthis invention for bringing each heater in the multiple zones to a setpoint temperature of the mold of FIG. 1.

FIG. 11B is a time vs. temperature plot of the prior art start-upcontrol for bringing multiple heaters to a set point temperature.

FIG. 12 is a flow chart of the software for accessing the IPM of FIG. 1from a remote location.

FIG. 13 is a schematic view showing a closed loop control of a chillingfunction for the mold of FIG. 1, and the use of the IPM with otherequipment relating to the injection molding operation.

Appendices A-1 through A-4, which are incorporated herein by reference,are descriptions and software listings of a preferred embodiment of thepresent invention.

DESCRIPTION

Overall Apparatus

Referring to FIGS. 1 and 2, there is shown the general arrangement of amold apparatus 10 according to the present invention. The mold apparatus10 comprises multiple molding stations, each having a hot runnerinjection mold 12. As is typical in the industry, the mold 12 comprisesa manifold backing plate 14, a core retainer plate 16, which capturesand holds in alignment a cavity plate 18, a mold cavity insert 19 in thecavity plate 18, and a manifold plate 20. Integral with the mold 12 isan interactive process manager (IPM) 22 connected via a datacommunication/power cable 24 to a communication/power unit 26 which, inturn, is connected to an AC power source through a power cord 28.

The IPM 22 includes a housing 30 affixed to the mold, optionally, by legelements 32 that support the IPM above the mold 12, providing a heatinsulating air space 34 to minimize any heat transfer from the IPM 22 tothe mold 12. Unlike prior art controllers, the IPM is directly attachedto the mold 12, and only a single connection needs to be made with thecable 24 to the communications/power unit 26.

Although the concept of an integrated IPM 22 is described herein interms of a molding process in a hot runner injection mold, it isintended that it can be applied to a wide range of apparatus whosesuccessful operation depend on measurable parameters that meet specificoperating conditions important to turning out an acceptable end product,and where there is a need for this system to correct itself withoutoutside intervention. This includes cold runner injection molds, spinmolds, blow molds, and the like.

The communication/power unit 26 is equipped with a PC (personalcomputer) terminal 36 which can be input information from a local PC 38,or optionally a user can communicate through a remote computer connectedto a phone line connection 39 for an internal modem 40 (FIG. 8). Thecommunication unit 26 additionally is equipped with status indicatorlamps 42, circuit breakers 43, and communication connection 44 foroptional connection to a network controlling other apparatus.

As shown in FIGS. 2 and 3, the IPM 22 is integral with the mold 12. Thecore retainer plate 16 and the mold cavity insert 19, when alignedtogether, form a mold cavity 46, into which molten resin is injectedthrough an opening 45, to form a molded component 47. The core retainerplate 16 and the mold cavity insert 19 contain the male and female moldportions 16 a, 19 a, respectively, that shape molded components 47. Themanifold plate 20 and the cavity plate 18 are constructed to receive ahot runner injection nozzle body 48 equipped with a nozzle tip thatcommunicates with the mold cavity 46 through an injection gate (notillustrated). The plates 16 and 18 are restored in the proper spacedalignment after the mold separates to eject each molded component 47.The manifold plate 20 is formed with a hot runner manifold 50 forsupplying resin melt to the injection nozzle body 48.

Monitoring Irregular Molding Condition

FIG. 3 is a sketch illustrating monitors or detectors for controllingcertain irregularities that can occur within the mold. With reference toFIG. 3, a transducer 52 is fixed within the insert 19 for sensing thedegree of movement between the parting line formed by the mold surfaces19 b and 16 b. Alternatively, the transducer can be fixed to the coreretainer plate 16. The quality of molded components 47 ejected from themold cavity 46 requires that the cavity insert 19 and the core retainerplate 16 be restored exactly to their original position after every timethe mold opens to eject molded components 47 and is subsequentlyrestored to its operating condition. Preferably, the transducer 52 is amicroswitch force sensor manufactured by Honeywell of Minneapolis,Minn., FS series device. The transducer 52 senses engagement of aprojection 16 c on surface 16 b with a recess 19 c in surface 19 b.Cables 56 connect the transducer 52 to the IPM 22, which monitors thecavity plate line movement of the mold. In the event misengagement ormisalignment is detected, the transducer 52 generates a millivolt signalthrough cables 56 for pick up by the IPM 22, which triggers a defaultsignal in the communication unit 26 causing an alarm, and optionally,shutting down the affected mold. Also, an alarm in the IPM 22, asdescribed below, is triggered.

It can be important to measure the aggregate weight of the components 47as they are ejected from the mold cavity 46 to make certain that themold is operating properly. For this purpose, a receiving tray 58 isplaced in the gravitational path of the components 47. The tray issupported on an electronic scale 60 that weighs components ejected fromthe mold. A conveyor 62 removes finished components 47 to an appropriatecollection station. Other types of removal and collection devices can beemployed in place of the conveyor 62 such as, for example, a roboticsweep arm (not illustrated) to collect finished components. The signalgenerated by the electronic scale 60 represents the aggregate weight ofthe components. The signal is transmitted to the IPM 22 by a cable 63,where it is received in memory in the IPM. In the event the aggregateweight of the molded components 47 is outside a preselected weightparameter, as recorded in memory, an alarm is activated in thecommunication unit 26, and optionally, the affected mold is shut down.

The condition of the mold after each hardened molded component 47 isejected is important, because it is necessary that the mold be ready toreceive the molten resin for the next molding operation. A video camera64 is positioned between the parting line surfaces 19 b and 16 b foridentifying any failure of the components 47 to clear the mold cavity 46after they are hardened or set. In particular, should a component failto clear the cavity, the video picks up an obstruction, showing thatsurfaces 19 b and 16 b of the mold have one or more components stuck ina mold cavity. This signals an alarm, and optionally shuts down themold. The video camera can also be used to remotely inspect the qualityof the ejected components.

The instantaneous recognition by the IPM of any one of these irregularconditions can shut down the mold 12, and/or can enable an operator totake timely action to correct irregularities. This minimizes costlyproduction of defective parts, which may otherwise go undetected for aninordinate length of time.

Temperature Control

Good temperature control of the mold requires control of the temperatureof each molding station. Although the following discussion is directedto control of a single molding station provided with multiple heatersand multiple sensors, it should be understood that the IPM issimultaneously controlling the temperature of all molding stations ofthe mold 12, where each molding station can have one or more heaters andone or more sensors.

The quality of the molded components 47 depends, in part, on the abilityof the system to maintain correct temperature. For this purpose, the hotrunner manifold 50 typically has one or more heaters. For example, apair of heaters 66 a and 66 b are provided in the resin feed structureat one end, and heaters 68 and 70 are located where the resin feeds tothe hot runner nozzle body 48. The actual location and size of theheaters used depends on the type of resin employed and details of themolded component. A chilled water system (see FIGS. 2 and 13) includes awater chiller 71, a chilled water inlet line 72 a, a chilled wateroutlet line 72 b, and a chilling fluid path 72 c in the mold for keepingthe cavity plate 18 and insert 19 at a desired temperature. The thermaleffect of the heaters 66, 68, and 70 on resin passing through the hotrunner manifold 50 and the hot runner nozzle body 48 is measured bysensors 74 and 76, typically thermocouples, strategically positioned inthe system. The thermal effect of the chilled water system is measuredby a sensor 77, typically, a thermocouple sensor. The sensor 74 is usedto control heaters 66 a, 66 b, and 68; the sensor 76 is used to controlthe heater 70; and the sensor 77 is used to control the outlettemperature of water from the chiller 71. Thus, each molding station canhave multiple zones, where each zone has a sensor and one or moreheaters or chillers responsive to the temperature sensed by the sensor.

The heaters 66 a, 66 b, 68, and 70 are powered by alternating current,as described in detail below, by a power drive/current sensor module 78,to which they are connected by heater cables 80, 81, 82, and 83,respectively. Each sensor 74, 76, and 77 is connected to the IPM 22 bysensor cables 85, 86, and 87, respectively. Each sensor generates ananalog signal which is transmitted by the sensor cables to the IPM 22,which is equipped with a processor for processing the measured signals,and thus controlling the power driver/current sensor module 78 and thechiller 71, in a closed loop system, as described in detail below.

Referring to FIGS. 2, 4, 5, 6, 7A, 7B, 7C, 7D, and 7E, the IPMcontroller 22 includes a first, data-receiving processor module 88 (alsoreferred to as a measurement processor module), and a second controlprocessor 90, a modem 92, a failure alarm 94, and the powerdriver/current sensor module 78. The failure alarm 94 can be triggeredby the IPM 22 sensing of the irregular conditions in the mold, asdiscussed above. The data-receiving processor module 88 receivestemperature measurements and current measurements, as described indetail below. In this present invention, the control and measurementfunctions are split to different processors, because the measurementfunction is CPU intensive requiring much processor capability. It hasbeen found that this is a less expensive control system than to use asingle processing unit for both control and measurement functions.

Because measurement of temperature and current are CPU intensive, for alarge number of zones, the data-receiving processor module 88 canrequire multiple processors. Thus, for the exemplary apparatus, shown inthe drawings, which has 96 zones, there is a first measurement processor88 a that is used for zones 1-48, and a second measurement processor 88b that is used for zones 49-96. For an exemplary zone having one heater(such as heater 70) and one temperature sensor (such as sensor 76), eachmeasurement processor 88 a and 88 b is receiving data signals from 96sources (2×48). Each measurement processor 88 a, 88 b providesmeasurement data to the control processor through cables 89 a and 89 b,respectively.

As shown in FIG. 4, the communication unit 26 is separate from the IPMunit 22. In addition to the components described earlier, thecommunication unit 26 has a step down transformer 98 that reduces theinput voltage, typically 240 volts, to a lower voltage, preferably about24 volts. The reduced voltage from the transformer 98 is fed to acontroller interface circuit 100 that supplies the reduced voltage powerto the IPM 22, including the modem 92, the failure alarms 94, and thepower driver/current sensor module 78.

The thermocouple sensors 74, 76, and 77 (FIG. 2) generate analog signalsalong the cables 85, 86, and 87. The signals are input throughcorresponding noise filters 102 a, 102 b, and 102 c (FIG. 5), and into amultiplex circuit 104. A suitable multiplex circuit 104 is availablefrom Analog Devices of Norwood, Mass., Model ADG 409BN. The filters 102filter out any noise spikes introduced by the 240 volt supplied to theheaters, and serve to avoid disruption of the millivolt signalsgenerated by the thermocouple sensors.

The millivolt signals from the multiplex circuit 104 are fed to firstand second sequential OP amplifiers 106 and 107, counted and digitizedin an AD converter. The first OP amplifier 106 amplifies the low levelthermocouple signal. The second amplifier 107 is referenced to athermocouple cold junction circuit 112. The second OP 107 amplifier addsthe cold junction compensation sign from the cold junction circuit 112to the thermocouple signal. This is necessary because thermocouples donot measure absolute temperature. They only measure the temperaturedifference between the end of the thermocouple cable in the mold, andthe end of the thermocouple which is connected to the measurementprocessor module 88. The cold junction compensator 112 gives a signal toadjust for the “ambient” temperature of processor module 88. A suitablefirst amplifier 106 is available from Linear Technology, Model No.LT1100; a suitable second amplifier 107 is available from Analog Devicesof Norwood, Mass., Model OP27. A suitable AD converter is available fromNational Semiconductors, Model ADC 1251C1J. The AD converter 108 isprovided with a precision voltage reference circuit 114.

With reference to FIGS. 7A-7E, the power driver/current sensor module 78serves two functions. First, it senses the current flowing to eachheater, and transmits the sensed data to the IPM 22. Second, it respondsto instructions from the IPM 22 to provide the necessary amount ofcurrent to each heater. In particular, the power driving unit 78 inputsanalog current signals to the processor module 88. The current for eachheater is sensed by a corresponding transformer 118 (FIG. 7) as part ofthe power driver/current sensor module 78. The signals pass throughconductors 116 for zones 1-48 and through conductors 117 for zones 49-96to a multiplex circuit 119 (FIG. 5). A suitable multiplex circuit 119 isavailable from Analog Devices, as Model DG408DJ. The current signalspass through an OP amplifier 120 (such as Analog Devices OP27), and arecounted and digitized in an AD converter 122 (such as NationalSemiconductors Model ADC 1251CIJ), and input into a microprocessor 123.A suitable microprocessor 123 is available from Dallas Semiconductor ofDallas, Tex., as part number DS87C520, which operates at about 11 MHZ.The measured temperature and current data from the processors 88 a, 88 bare input to the control processor 90 through cables 89 a and 89 b,respectively. The control processor 90 evaluates the input data anddetermines the power requirements necessary to maintain the zones attarget operating conditions.

FIG. 7E shows a circuit diagram representing a portion of the powerdriver unit 78 for one zone. In practice, a typical power driver unit 78includes a series of the circuits shown in FIG. 7C, one for each heater.AC power is provided over AC conductors 127, 129, across which,connected in parallel, is a power driver switch 126 (also referred to asa power triac). The current transformer coil 118, in series with thepower triac 126, measures current passing to the heater 66. A zerocrossing interrupter 128 (FIG. 8) receives a zero crossing signal from azero crossing detector 130 after passing through a band pass filter 132(FIG. 8), which screens out unwanted noise from the AC power (FIG. 8).The power triac 126 is energized for applying power to the heater 66 inaccordance with signals from the control processor 90 passing through anopto driver 136.

Referring to FIGS. 7A, 7B, 7C, and 7D, there is shown the structure of atypical power driver unit 78. As shown the power driver unit 78comprises a circuit board 137 having a top circuit 138 and a bottomcircuit 140 which are assembled back to back, and include the optodriver 136 and the power triacs 126.

AC (alternating current) power is input to the power driving unit 78 atterminals 142 and 144. Copper bus bars 147 and 148 on opposite sides ofcircuit board 137 connect to the input side of the triacs switches 126located in the bottom circuit 140. The opto triac drivers 136 areconnected to the bus bars 147 and 148 and the triacs 126 through anarray of resistors 150. Heavy bus bars 146 and 149 on opposite sides ofthe circuit board 137 serve as a common return path to the AC terminal144 directed to heaters through connectors 154. The conductive stripsare formed from copper sheets having a weight basis of 4 ounces persquare foot. It has been found that this weight of conductive copperstrip minimizes the amount of heat generated in the circuits 138 and 140of the power driver unit 78.

Current is conducted from bus bars 147 and 148 through an activatedtriac 126 through a wire 156. The current passes through the sensingtransformers 118 to connectors 154 which pass the current to the heaters66 (FIG. 7C), and is conducted via the bus bars 146 and 149 to theterminal 144. Connector 160 receives the heater on-off signal from thecontrol processor 90 and connector 162 sends the current data to thedata-receiving processor 88. Aluminum bracket 164 serves as a mountingbracket for the power driver unit 78, and as a heat sink for the powertriacs 126.

The power driver unit 78 is equipped with the elements (eight optodrivers 136, eight power triacs 126, and eight transformers 118)necessary to provide power to eight zones. The power driving unit can berepeated or expanded to accommodate additional zones. The eight zonescan be for one molding station, or more typically, multiple moldingstations.

Referring to FIG. 6 and Appendix A1, there is set forth the flow chartof a PID that determines the integral error and calculates the powerrequirements for each zone. The computations in processor 90 areperformed according to a PID (“Proportional Integral Differential”)which has been modified according to A1 to avoid the risk of computeroverflow. Overflow could result in applying excessive power to theheaters, causing production of defective parts, and possibly damage tothe mold apparatus.

In each duty cycle (also referred to as a polling cycle) of theprocessor 90, all temperature sensors 70 and 74 and all current sensingtransformers 118 are sampled at least one per second, and preferablyevery half second. In a first microprocessor step 264 (A1-53), index iis set to one of the zones, starting with i=0 for the first zone. Aninitial reading step 265 measures the present temperature of the zone PT(A1-61). In the next step, step 266, the temperature error (TE=PT−Targettemperature) (A1-69) is determined. The difference between the presenttemperature PT and the Target temperature is the operational referenceparameter. The differential error DE A1-70 is calculated in step 267,taking the difference between TE and the previous TE measured in aprevious polling cycle. Instep 268, A1-71, the sum integral error,(ΣE[i]=E[i]+TE), is calculated.

At this point in the PID, limits on the sum integral error are imposedin order to avoid arithmetic overflow of the calculated power values,which could call for excessive damaging power input. Steps 269 and 270prevent the power supply from applying a value of CP[i] greater than 100percent. Steps 269, 269A, 270, and 270A (A1-76, 77, 79 and 80) placelimits on the integral error between a positive maximum value and anegative maximum value to avoid overflow.

Step 271 (A1-86) performs a power requirement calculation according tothe following equation:

CP[i]=[(KP[i]×TE+KD[i]×DE)×1000+Kl[i]×ΣE[i]]/1000  EQUATION 1

where: KP[i] = experimentally determined proportionality constant forPID algorithm for heater [i] KI[i] = experimentally determined integralconstant for PID algorithm for heater [i] TE = proportional temperatureerror calculated in step 266 DE = differential temperature errorcalculated in step 267 KD[i] = experimentally determined differentialconstant for heater [i] ΣE[i] = integral temperature error for zone idetermined in step 268, subject to the limits of steps 269, 269A, 270,and 278

The value for each constant “K[i]” is determined empirically byobserving the rate at which each respective heater [i] is brought to itsoperating temperature. Multiplying the proportional and differentialterms by 1000, (A1-86) simply allows the PID constants KP, KI and KD tobe conveniently expressed as reasonably sized integers. Havingestablished the limits for the integral error, the calculated powerCP[i] is calculated according to the PID formula shown in step 271,(Equation 1).

If the value for CP[i] is less than 0, it is set to zero in steps 272and 272A (A1-91, 92). If CP[i] is not less than zero, and is greaterthan 1000, it is set to 1000 in steps 273 and 273A (A1-88, 89). Step273A is equivalent to a power value of 100%, because power is calculatedin units of 1/10%, so 1000 means 100.0%. Calculations are done asintegers for processing speed. The temperature error (TE) and measuredRMS heater current, discussed hereinafter in connection with theapplication of power, are compared to limits in steps 274 and 275 todetermine if there is a temperature or current alarm condition, forpossible activation of alarms.

For example, a temperature alarm is activated in step 274 (A1-82) if theset temperature is exceeded by a predetermined value, such as by fivedegrees. Similarly, a power current alarm is activated at step 275(A1-82), if the current level is outside a specified range.

The power level CP[i] in the range of 0-1000 is converted in step 276 toPower [i] in the range of 0-128 for later use by the modulo “N” rateaccumulator. In the next step 277 (A1-97), the temperature error for thezone is reserved for use in the next polling cycle (A1-53). The laststep 279 (A1-53) determines whether the program has polled all of thezones during the duty cycle. If it has, polling is repeated startingwith zone 1. If it has not, polling proceeds to the next zone.

Application of Power

Returning now to FIG. 5, the measurement processors 88 a and 88 breceive digitized signals at the rate of 120 measurements per one halfsecond duty cycle from each current sensor and eight measurements perone half second duty cycle for each temperature sensor. This imposes asignificant calculation burden on the microprocessor 123 of eachmeasurement processor to calculate RMS current and temperature values.During each 0.5 second PID duty cycle, each current sensor 118 (FIG. 7c)is read 120 times, that is, four equally spaced readings for each cycleat 60 Hz. For 50 Hz, four evenly spaced readings occur every {fraction(1/50)}th second.

The calculation of the RMS (“Root Mean Square”) value for temperatureand current values, requires that the calculation be accomplished morerapidly in order that the system keep up with the volume of data. Inorder to deal with the volume of data, the following equation for N bitdata (Appendix A4), calculates the sums of each of the 2N bit partialproducts and combines these partial sums into a 4N bit final sum:

RMS=SUM[(AL×AL)]+SUM[(AH×AL)]×2×2^(N)+SUM[(AH×AH)]×2^(2N)  EQUATION 2

where AL is a low N bits for the 2N bit data, and AH is the high N bitsof the 2N bit data. These sums can be represented in 3N bits without thepossibility of overflow provided the sum is no more than 2^(N) readingsper measurement cycle.

Processor 90 determines when to trigger the power switch unit 78determined by a cycle skipping algorithm. Power is applied to each zoneby turning on the triac 126 (FIG. 7B) of the power driving unit 78 forone or more complete cycles of the 50/60 Hz AC wave form. Thecontrolling software always turns “on” and “off” at the zero crossingpoints of the AC wave form which minimizes both generated noise andpower dissipation. The modified PID algorithm in the processor operateson a duty cycle of 0.5 seconds which is thirty full cycles of the ACwave form. It is desirable to control the power level to a much finerresolution than once in thirty discrete steps. This is accomplished byaveraging out the CP[i] over a period of time by applying a waveaccumulator function to the average value of the applied power, CP[i],(Equation 1), over the period of measurement to determine when toactivate the triac.

Referring now to the block diagram of FIG. 8, there is illustrated thestructure of the control processor 90 which includes a microprocessor399. The microprocessor 399 computes the timing when the CP[i] is to beinput to the heaters. The processor 90 includes the microprocessor 399and CMOS standard random access memory 411 (“CMOS SRAM”) that storesoperational parameters, such as target temperatures, heat currentlevels, and constants in the above equations. The microprocessor 399receives input for component weight control, mold spacing, and componentejection from the mold. The CMOS SRAM 411 is available from Hitachiidentified as Model HM 628128. A suitable microprocessor is availablefrom Intel, Santa Clara, Calif., and operates at 25 MHz.

The parameters in memory 411 can be modified by inputting changes fromthe laptop computer 38 to the microprocessor 399. A flash memory 414makes possible altering the software recorded in SRAM memory 411.Changes and modifications can also be made by the phone connection 39 tothe modern 40. A calendar clock 416 provides all the time dependentfunctions as the start-up control.

The processor 88 sends its RMS current measurement (as shown in AppendixA4) and average temperature measurements to the microprocessor 399 ofthe control processor 90, which performs the PID calculation of FIG. 6(as specified in Appendix A1).

The calculations made by the microprocessor 399 determine the 3 phasepower to be delivered to the heaters by the power drivers 78. The powerdrivers 78 are controlled by on/off signals output by microprocessor 399to buffer drivers 519. These on/off signals are synchronized with thezero crossings of the 3 phase AC waveforms. The zero crossing points aresensed by the zero crossing detectors 130 on the power drivers 78. Thesezero crossing signals are first filtered by the 60 Hz bandpass filters132 to remove any unwanted noise, and then passed through the zerocrossing interrupter 128 to generate zero crossing interrupts to themicroprocessor 399. In FIG. 8 there are depicted buffer driver outputsfor 12 power drivers, each of which can control power to 8 heaters, fora total of 96 heaters.

There is shown for each heater [i] a software flow chart FIG. 9,Appendix A2, which provides a rate accumulator function for controllingthe time when the CP[i] current is to be applied a heater [i]. A firstdecision 322 (A2-36,70,104) initiates the accumulator calculation. Instep 323, the power, P[i], applied to the heater [i] is compared to zero(A2-39, 44, 73, 107, 78 and 112). The rate accumulator function is basedon modulo arithmetic which means that the numbers are constrained to afinite range. For the purposes of the instant software program, aseven-bit binary number is chosen in the numerical range of 0 to 127 asthe modulo number. The accumulator control involves a timing or ratefunction which depends on the cycles that comprise the AC wave form.Every zone is polled 60 times per second, i.e., once every cycle of theAC wave form. In view of the fact that the CP[i] is already determined,the question is timing the input to the heater. The rate accumulatorperforms according to the following equation:

Modulo 128(ACCUM[i]+CP[i])<=ACCUM[i]  EQUATION 3

where Modulo 128 is a modulo arithmetic number.

For purposes of illustration, if it is assumed that the power is beingprovided to the heater [i] at 25% of capacity, the rate accumulatoractivates the opto drive 136 at the zero crossing of the AC cycle, andthen turns it off for the next three cycles, which provides a 25% powerinput. Using a 7 bit modulo number of 128, accumulator overflow occurswhen the accumulator exceeds 127, thereby firing the opto drive 136 atthe zero crossing point, closest to that signal.

Referring to decision steps 324 and 325 (A2-40, 45, 74, 79, 108 and113), the triggering calculation is made by adding to accumulator [i]the power percentage, power [i] (determined in step 276) multiplied bymodulo number 128 (7 bit number). When the accumulator value [i] is lessthan or equal to the old accumulator value, indicating an overflow ofthe modulo 128 accumulator, the triac is fired for one cycle as shown instep 326 (A2-40, 45, 74, 79, 108 and 113). If the decision in 326 is“no” for the reason that the new accumulator value is greater than theold accumulator value, then in a step 327 (A2-41, 42, 75, 80, 109 and114) the process merely advances to the next heater (A2-36, 70 and 104)since no power input is required. The next decision 329 is to determinewhether the zone just measured completes the cycle (A2-36, 70, 104). Themicroprocessor 399 (FIG. 8), in response to the decision in 326, sends asignal to the switch 136 to fire the appropriate power input to theappropriate heater.

The processor 90 is connected to certain additional control elementssuch as the static random access memory (SRAM) 411, the flash memory 414that enables making changes to the set of operational referenceparameters, the calendar clock 416 and the battery and power failuredetect circuits 418. All the calculations performed in microprocessor399 are stored in the CMOS SRAM 411.

Application of Chilling Water

The same general algorithm is used for control of the chiller 71, wherethe parameter monitored and controlled is temperature of the outputwater from the chiller 71 rather than power delivered to the heaters.The chiller is operated at substantially constant pressure andsubstantially constant flow rate, with only the output temperature beingvaried in response to the temperature sensed by the sensor 77. As shownin FIG. 13, a temperature sensor 285 detects the temperature of outputchilled water and sends an analog signal to the IPM 22, where the signalis digitized and processed just as the current signals sent by thetransformers 118 are processed.

Start-Up Control

The IPM 22 is effective in managing the start-up of a multiple zoneapparatus from a nonoperating condition bringing it to full operation.Referring to the flow chart of FIG. 10 and the plots of time v.temperature in FIGS. 11A and 11B, there is shown the start-up system ofthe IPM 22. The start-up of a new operation is best carried out witheach of the plurality of zones/heaters progressively heating up from astarting state and reaching an operational state at the same time.Careful start-up avoids damaging the heat sensitive materials within thesystem.

A preferred start-up procedure according to the present invention willnow be discussed. The calendar clock 416 of the processor 90 is set atzero and a ramp up time (RUT) is programmed into the microprocessor 399to correspond to the longest period of time required for a zone heaterto reach its operational temperature. It will be understood that eachheater has a different thermal mass which results in different rates fora heater to heat the corresponding mold portion and reach its operatingtemperature.

Referring now to FIG. 10 and Appendix 3, the system in the first step500 (A3-31-35) sets a time t to 0 in the calendar clock 416 in thecontroller 90 and a value for RUT (ramp up time) in step 502 (A3-70-30),which is based on the longest heat-up time for all zones. The heat-uptime is calculated based on the time constants of the heaters and therequired temperature rise. Longer time constants and larger temperaturerises result in longer calculated heat-up times while shorter timeconstants and lower temperature results in shorter warm-up times. All ofthe zones in step 504 (A3-10), are set with an initial temperatureIT[i], which represents the present temperature of the system PT[i] atstart-up.

In step 506 (A3-39, 40) the software refers to a look-up table whichdefines the shape of the warm-up curve. The software selects a TableIndex, TI, in terms of the current time multiplied by a factor of 1000and divided by the RUT. The look-up table provides an incremental steprate at which power is applied to each heater so that all heatersproceed to follow a warm-up curve that approximates the sinusoidal curveshown in FIG. 11A so that all zones reach their respective desiredstart-up temperature at about the same time. The curve S represents thetemperature profile of the heater with the smallest thermal mass and Lrepresents the profile of the heater with the largest thermal mass. Bothcurves S and L follow the target curve T, determined empirically, whichindicates that at no time during the start-up, neither curve S or Ladvance to the target temperature before the other. The rise of curve Lgradually slows and levels off at the end of the warm-up cycle at theposition C. The information from the look-up table is utilized to setthe operational zone temperature calculated according to the equation:

IT[i]+(set point[i])−IT[i])×interp/1000.  EQUATION 4

Process step 507 (A1-66, 67) establishes an intermediate setpointtemperature between the initial temperature IT[i] and the final setpoint[i] established by Equation 4 above. All zones follow the same smoothtemperature rise S curve determined by the lookup table, so that allzones arrive at the final setpoint temperature at the same time.

In step 508 (A1-41) it is determined whether any zone has exceeded, oris lagging behind, its set point temperature. If no zone is lagging,then the clock 426 indexes the application of CP to the next step 509(A1-42). If a zone is lagging, then the temperature has deviated too farfrom the warm up curve and the progression of the start-up is halted.Step 510 (A1-38) determines if the elapsed time t has exceeded the RUTindicating the end of the start-up cycle.

The curves in FIG. 11B are a plot of the time and temperature functionof the known prior art start-up techniques for target temperature T′ andthe smaller heaters, S′, and larger heaters, L′. This prior art approachis to have the smaller heaters reach the target temperature before thelarger heaters. In fact, curve L′ is at the target temperature while S′is still at a very low temperature at X on the time coordinate. It willbe observed that the curve L′ overshoots the target T′ for the reasonthat it is still being applied power and it undershoots the set pointtemperature before it reaches the RUT at the end of the cycle. Unlikethe start-up system shown in FIG. 11B, the technique of the presentinvention results in all zones reaching their start-up temperaturessimultaneously.

Pressure Control

The transducer 52, shown in FIG. 3, was described above with regard tomaking certain that the cavity plate insert 19 and the core retainerplate 16 are restored to their original position after every time themold opens to eject molded components. The transducer 52 can also beused to monitor the injection process to make certain that there is notexcessive pressure in the mold cavity with the result that the cavityplate insert 19 and the core retainer plate 16 separate at the partingline, with resultant formation of flash. If during the resin injectionphase, the resin pressure is too high such that the pressure device(such as mechanical or hydraulic pressure) used to keep the mold partstogether is overcome, then the transducer 52 detects lack of engagementbetween projection 16 c and the recess 19 c, and transmits a signal.This signal can alert the operator to the need to manually increasepressure, and/or sound an alarm, and/or be incorporated into a closedloop system where pressure provided by the mold hydraulic system 299(FIG. 3) is automatically increased under control of the IPM 22.

Remote Access Control

The system can be controlled by an operator from a remote location asshown in FIGS. 8 and 12 by accessing the modem 40. Subject to anappropriate password by user input 602, a remote PC (not shown) cancommunicate with the controller when the modem is detected in steps 603a and 603 b. The operator has access to a menu 604 which can selectseveral system functions for information or modification. There is a login/log out function 606. The operating conditions which are then ineffect at the time the operator accesses the PC can be changed by userinput 602. When the parameters of the system need revision or changing,the operator can access an edit function 610, which can make anyrequired changes to the conditions in the processor 90. Function item608 can be used to view current operating conditions and settings.Function item 612 can be used to start or stop heaters. Function item614 can be used to save or retrieve a complete set of operationalparameters (called a “recipe”) from memory. The remote access program isconcluded by logging out of the system, or by loss of carrier to themodem.

Thus, through the IPM it is possible for the mold to communicateremotely the condition of the mold, including operating conditions,faults, errors, safety, and any diagnostics.

Interface with Other Apparatus

An advantage of the IPM 22 is that it can be used to interface the mold12 with other support equipment, as shown in FIG. 13. This can includechillers, blenders, robots, parts handling equipment, other molds, andinjection molding machines.

Exemplary Advantages

The present invention has significant advantages. For the first time, amold is able to self control, without the use of outside controllers,its functions, pressures, movement, safety, ejection, and temperatures.The IPM 22 can be used for controlling each step of the molding processat each molding station. Moreover, chilling water and the pressure atthe parting line can be controlled as part of a closed loop function,without operator interaction. Use of the video camera and scale allowremote monitoring of product quality. Furthermore, setup is simple andfast, with a single plug providing the input power and signals, andoutput signals to the mold, including through the IPM 22. Otheradvantages are associated with the improved start-up procedure, and fastresponse time and accurate control achieved with the multipleprocessors.

Although the present invention has been described in considerable detailwith reference to certain preferred versions thereof, other versions arepossible. Therefore, the spirit and scope of the appended claims shouldnot be limited to the description of the preferred versions containedherein.

All features disclosed in the specification, including the claims,abstracts, and drawings, and all the steps in any method or processdisclosed, may be combined in any combination, except combinations whereat least some of such features and/or steps are mutually exclusive. Eachfeature disclosed in the specification, including the claims, abstract,and drawings, can be replaced by alternative features serving the same,equivalent or similar purpose, unless expressly stated otherwise. Thus,unless expressly stated otherwise, each feature disclosed is one exampleonly of a generic series of equivalent or similar features.

What is claimed is:
 1. An apparatus comprising: (a) multiple zones, eachzone having at least one heater and at least one temperature sensoroutputting a temperature indicating signal; (b) a power source providingpower to the heaters; and (c) a controller for controlling thetemperature of at least some of the zones, the controller comprising:(i) a data-receiving processor for receiving the temperature indicatingsignal from each temperature sensor, computing RMS values correspondingto measured heater currents and computing the average of the temperatureindicating signals; (ii) a power driver comprising respective on/offswitches for controlling power to each heater, respective currentsensors for measuring the heater currents in each heater, and a zerocrossing detector for triggering the on/off switch; and (iii) a separatecontrol processor for receiving data from the data-receiving processor,storing a selected range for the temperature in each zone, performing aPID calculation of the amount of power required to maintain thetemperature of each zone within a selected range based on the valuescomputed by the data-receiving processor, and for controlling the amountof power provided to the heaters in response to the data received fromthe data-receiving processor, the separate control processor comprisinga zero crossing interrupter interposed between the zero crossingdetector and the on/off switches to provide only complete AC cycles tothe heaters; whereby each zone is maintained within the selected rangeof operating temperatures.
 2. The apparatus of claim 1, wherein eachzone has at least one chiller, the apparatus further comprising a sourceproviding a cooling medium to the chillers, the separate controlprocessor also controlling the amount of chilling medium provided to thechillers in response to data from the data-receiving processor.
 3. Theapparatus as claimed in claim 1 or 2 wherein the controller comprises aclosed loop feed back circuit.
 4. The apparatus of claim 1 wherein thetemperature indicating signals are analog and the controller comprisesat least one analog to digital converter for converting the temperatureindicating signals to digital signals.
 5. The apparatus of claim 1wherein the apparatus is a mold having multiple injection stations, andthe controller is in a housing, the housing being mounted on the mold.6. The apparatus of claim 2 wherein the apparatus is a mold havingmultiple injection stations, and the controller is in a housing, thehousing being mounted on the mold.
 7. The apparatus of claim 5comprising an insulating air gap between the housing and the mold. 8.The apparatus of claim 1 or 2 comprising at least 48 zones, and whereinthere are at least two data-receiving processors, each receivingtemperature indicating signals.
 9. The apparatus of claim 1 wherein theapparatus comprises an injection mold having multiple injection zones,and each injection zone is used to form molded components, and theapparatus comprises respective transformers for measuring the current toeach heater, and an alarm responsive to an out of control conditioncomprising an abnormal heater current if present and, if present, acondition selected from the group consisting of incomplete ejection of amolded component from the mold, irregular spacing between moldcomponents, and incorrect weight of molded components.
 10. The apparatusof claim 9 further comprising a switch responsive to the alarm forautomatically shutting down the apparatus in response to the out ofcontrol condition.
 11. An apparatus comprising: (a) multiple heatedzones, each zone having a target temperature and (i) a temperaturesensor providing a temperature indicating signal; and (ii) a heater; (b)a power source providing AC current to each heater; (c) a detectordetecting the amount of current provided to each heater; and (d) acontroller comprising: (i) a first processor for receiving thetemperature indicating signals; (ii) memory for storing the targettemperature for each zone; (iii) a separate second processor forcomparing the actual temperature of each zone against its targettemperature and for comparing the detected heater current with an alarmthreshold; and (iv) an output signaler for regulating the percentage ofcomplete current cycles provided to the heater for each zone, the outputsignaler being responsive to the comparison of heater current with thealarm threshold for detecting an alarm condition when any of the heatercurrents exceeds the alarm threshold.
 12. The apparatus of claim 11wherein the apparatus is a hot runner injection mold.
 13. The apparatusof claim 12 wherein the controller is mounted on the mold.
 14. Theapparatus of claim 11 wherein the second processor determines the numberof current cycles required to achieve the target temperature andcompares the determined number against the number of current cyclesactually being provided to the heater.
 15. The apparatus of claim 11further comprising a switch responsive to the second processor forautomatically shutting down the apparatus in response to the alarmcondition.
 16. The apparatus of claim 11 comprising an on-off switch foreach heater, the switch being responsive to the output signaler.
 17. Theapparatus of claim 16 wherein there are a plurality of zones, and thecontroller comprises at least two printed circuit boards, one circuitboard containing the second processor and the other circuit boardcontaining the switches.
 18. An injection mold comprising: (a) at least48 injection stations, each station having at least one heater and atleast one temperature sensor, each sensor providing a temperatureindicating signal; (b) a source of current for the heaters; (c) acontroller for controlling the temperature of the station, thecontroller comprising at least one processor for receiving thetemperature indicating signals from the sensors, and an on/off switchfor each heater controlling the amount of power going to each heater,the switches being controlled by the processor, wherein the at least oneprocessor is on a printed circuit board and the switches are on aseparate printed circuit board, the temperature signals from up to atleast 48 of the injection stations being received by a single processorat a rate of not less than one temperature indicating signal from eachtemperature sensor per second.
 19. The mold of claim 18 wherein theprocessor receives a signal indicating a measured amount of currentflowing to each heater.
 20. The mold of claim 18, wherein the controlleris within a housing, and wherein the housing is mounted on the mold. 21.The mold of claim 20, further comprising each station having at leastone cooling path and a source of chilling fluid for the cooling path;and the controller also controlling the temperature of chilling fluidprovided to the cooling path in response to the received temperatureindicating signals.
 22. A hot resin injection mold apparatus, the moldhaving at least forty-eight stations and at least one heater for eachstation, the apparatus comprising: (a) at least one temperature sensorin each station, each sensor outputting a temperature indicating signal;(b) an AC power source generating cycles of an AC wave form providingpower to the heaters; (c) a power driver comprising: (i) an on/offswitch for controlling the power to each heater; (ii) a transformer formeasuring the current to each heater; and (iii) a zero crossing detectorfor triggering the on/off switch; (d) a first processor for receivingthe temperature indicating signals comprising: (i) an AD converter fordigitizing the temperature indicating signals; and (ii) a firstmicroprocessor for computing the RMS of the measured current values andcomputing the average of the temperature indicating signals; and (c) asecond processor comprising: (i) data memory for storing a selectedrange for the temperature in each zone; (ii) a second microprocessor forperforming a PID calculation of the amount of power required to maintainthe temperature of each zone within a selected range based on the valuescomputed by the first microprocessor; and (iii) a zero crossinginterrupter for sensing the zero cross-over of the AC wave form andoutputting an interrupter signal for signaling the power driver toprovide only complete AC cycles to the heaters; whereby each station ismaintained within the selected range of operating temperatures.